Electrical transmission system



Oct. 23, 1934.

M. B. LONG 1,977,683

ELECTRICAL TRANSMISSION SYSTEM Filed May 22, 1926 lawn/0n Maw/Ce 5 [my by M4;

Patented Oct. 23, 1934 v UNITED STATES,

1,977,683 ELECTRICAL TRANSMISSION SYSTEM Maurice B. Long, Glen Ridge,

Western Electric Company,

N. 1., assignor to Incorporated, New

York, N. Y., a corporation ol Ncw York Application May 22, 1926, Serial No. 110,910 9 Claims. (01. 1787) This invention relates to electrical transmission systems and more particularly to systems for the transmission of pictures, scenes and the like to a distance.

It is the principal object of this invention to secure an improved method of employing a frequency modulated carrier for transmitting pictures, scenes and the like from a transmitting point to a remote receiving point.

Another object of the invention is to improve the efiiciency of transmission systems of this character by an improved method of guarding against the detrimental effects of current level variation.

According to the present invention there is provided a system in which a carrier current wave is modulated by a wave, representing a picture, scene or the like, in such manner that the frequency of the modulated carrier wave varies in accordance with the lights and shades of said picture or scene. The frequency modulated wave is then transmitted to the receiving station where it is employed to reproduce the picture or scene. Moreover, the invention contemplates a novel arrangement at the receiving station responsive to frequency modulated carrier current and serving to produce a current of amplitude which varies in accordance with the picture characteristics. It further contemplates means responsive to the incoming frequency modulated carrier wave for compensating for variations in the transmission level.

Other features and advantages of the present invention will appear from the following description and also from the appended claims.

The drawing is a circuit diagram illustrating a picture transmission system embodying the features of the invention.

The system disclosed herein is designed for the transmission of picture characteristics on the basis of frequency rather than amplitude modulation. To this end there is provided at the transmitting station, means for generating a sustained oscillatory carrier current together with means whereby the frequency of the carrier current may be varied or modulated in accordance with the tone values of a picture undergoing a scanning process. The frequency modulated carrier current is then transmitted over the line to the receiving station where it is impressed upon a circuit which is designed in such a manner that the loss in amplitude or attenuation is a rectilinear function of the frequency over acertain range which is here chosen as the operating range. It follows, therefore, that the output current of this circuit is one which varies in amplitude in accordance with the tone values of the transmitted picture.

,This variable amplitude current is then impressed upon the input circuit of a demodulating device. The current produced in the output circuit of this latter device is then applied to a well known light valve which serves to determine the exposure of a photo-sensitive surface to a beam of light for the reproduction of the picture.

The value of the normally constant amplitude of carrier current in the system may be changed due to disturbances in the system and particularly in the transmission line. In other words, the so-called transmission level may fluctuate. Such fluctuations, unless taken care of in some way, produce detrimental eifects in the picture at the reproducing station. These injurious effects are guarded against in the present system by introducing a gain control element consisting of a thermionic discharge device which responds only to changes in the direct current component of a current obtained by rectification of current flowing in the transmission line. In accordance with the response of the gain control element, the demodulating device may be caused to vary in a compensating direction to eliminate the effect of the transmission variation.

Referring to the drawing, there is shown a rotatable transmitting drum 1 mounted on a shaft 2 for rotary movement and also for axial movement. Upon the drum 1 a film 3 representing the picture is mounted so that as the drum rotates and traverses, a beam of light from the source 5, concentrated by the lens 6, is caused to scan the elemental areas of the film. The beam of light after penetrating the film 3, impingesupon the sensitive electrode of a photo-electric cell 4.

The photo-electric cell 4 is connected in circuit with a source of current 7 and the resistance ele ment 8. Thus, as,the tone values of the film vary, a variable current is caused to fiowthrough the resistance 8. The resistance 8 is also included in the input circuit of an amplifying device 10. The device 10 comprises a three-electrode thermionic discharge vacuum tube having a filamentary cathode 11 and a grid electrode 12 included in the input circuit. A battery 9 serves to maintain the grid 12 at the proper biasing potential. The output circuit of the tube 10 is connected across a choke coil 14 and a source of direct current 15. The battery 15 is for supplying the electron discharge between the cathode 11 and the anode 13, and the coil 14 is to prevent the presence of high frequency currents in this branch The transmission line L is also connected at of the circuit. the receiving station to the input circuit of a At the transmitting station, there is also progain control element 45 which consists of a vided an oscillation generator 18 which is in the thermionic discharge device of the well known form of a thermionic discharge vacuum tube. construction. The input circuit of the device 45 80 The generator 18 has an input circuit including i ude 8 fi am y at de 46 which is conthe filamentary cathode 19, condenser 27, a pornected to one terminal of the line L and a contion of the coil 23, resistance 17, and the grid trol electr d 47 wh ch is c n d th u h electrode 20. The output circuit of the oscillaslstences 41 and 42 to the other terminal of tion generator 18 includes the filamentary cathsaid line. A source of potential 44 included in 85 ode 19, battery 26, another portion of the coil circuit with the resistance 43 serves to maintain 23, and the anode 21. The coil 23 is inductively the G d electrode 7 at the desired b as coupled to a second coil 24, which is connected e a h u ut c cuit 01' the device 45 inacross the line L. The coil 23 is also connected eludes the a de 46 which is connected to one in parallel with a variable condenser 25. By ad- Side Of the Variable resistance 3 and the anode 90 justing the elements of the circuit 22, the device 43 Which is Connected through 8 Source O De 18 may be made to oscillate and produce a sus tial 50 to another terminal of the resistance tained oscillatory current of any desired normal 35. In pa a to the branch the Output frequency. cult just mentioned, there is another branch in- The output circuit of the device 10 is connected eluding the condenser 95 through a condenser 16 directly across the grid The Operation of the Sy t Will new be deelectrode 20 and cathode 19 of the generator 18. seribedwhen it is desired to transmit 8 D The condenser 16 prevents current from the batture, e fi m p s nt n the sam is mounted tery 15 from flowing 1n the input circuit of the upon the drum 1 a d ca ed y he optical generator 18. It will be noted that a shunt path system. The current flowing n e r stance 8 100 is provided across the output circuit of the device results in Variable Potential drop across Said 10 including the resistance 17, a portion of th resistance, thus varying the potential of the concoi123, and the c nden e 27 It, h been fou d trol electrode 12. Variations in the potential device 10, the frequency of the current produced ins Variations in the impedance of the Output 105 by the generator 13 may b made t ary directly circuit of the device 10. Accordingly, as above with variations in the impedance of said device explained, the frequency of the oscillatory c111" 10, due t th presence of t shunt circuit rent varies in direct relation with these variations tioned. In othe w rd if by means of the com of impedance. Hence, there is induced in the tr ectr 12 t impedance of the output secondary coil 24 of the tuned circuit 22, an os- 110 cuit of th ic 10 can be made t vary, the cillatory current the frequency of which is modufrequency f th oscillatory current produced by lated or varied in accordance with the variations the generator 18 will also vary in direct relation in the tone values of the picture being scannedthereto. For a further description .of a genera- This able frequency current is transmitted tor of this type referen e i made t patent over the line L and impressed upon the circuit 5 1,847,142, granted to L. J. Sivian, March 1, 1932. The circuit 28 Produces a current of very- At the receiving station, the transmission line 111% amplitude which causes the control electrode L is connected t th input end of an equalizing 37 to vary in potential in accordance with the or filtering circuit 28. This circuit may be of any variatlon in frequency of the current transmitted suitable construction and includes, for exampl Over the line. The variation in potential of the 120 the tuned loop circuits 29 and 30 and shunt electrode 37 causes the amplitude of the current densers 31 and 32. The purpose of the circuit flowing through the light Valve 51 to vary 28 is to receive the incoming current which has cordinsly- In response to this Variable amphasubstantially constant amplitude andavariable tude current the light Valve 51 governs frequency and t translate the Same into a amount of light reaching the reproducing record 5 rent having an amplitude which varies directly blank and ascordihgly determines the reproducas the variations in the frequency of the input tloh the plctul curre t, i It is well known that in carrier transmission At the receiving station there is also provided systems if a carrier current having a normal a thermionic discharge device 33 which may serve cohstaht frequency is modulated by a signal Wave as a demodulator or rectifier of the variable amof vanable amplitude there is produced a plitude current delivered by the filter 28. The suufing modulated wave the frequency of which device 33 comprises an input circuit including the valiles W the amplitude of the signal Wave filamentary cathode 36 connected to one terminal Whlch 15 made up of a component W havmg 60 of the circuit 28 and a control electrode 37 con- 3 frequency equal to that of the Carmel, together nected to the other terminal f the circuit with other component waves having frequencies A source f potential 34 is provided for main greater and less respectively than that of the taining the grid electrode 37 at a desired biasing carrier- The frequencies which are respectively potential. greater and less than that of the carrier are said 65 The output circuit of the device 33 includes to constitute the upper and lower side bands of the cathode 36, th anode 33, and a source of the frequency modulated carrier wave. It has space current 39. This output circuit is conbeen found that y mpressin a modulated ca nected to the vibratory string of a light valve rier Current Comprising these c mponen fre- 51, which may be of any well known construction. quencies u a t mi n disc r d v 01 The light valve 51 serves to control the inrat n as a d m du a a p o designed, tensityv of a beam of light shining from the source the output current of such device may be made 52 through the lenses 53 and 54 upon the surfaces to vary in proportion to the variation in ampliof a sensitized record blank which is moved in tude of the carrier current component and indea rotary and longitudinal direction by the pendently of the frequencies lying in the side 43 drum 55. bands. Stated otherwise, this means that the duce in its output circuit a (mi Wiltlli $116 intensity of which varies as the amplitude of the unmodulated carrier wave component in the transmission line. By means of this arrangement, there is produced in the output circuit of the tube 45 a current which is a measure of the fluctuations in the amplitude ofthe total current in the transmission system. Accordingly, any variations of level in the transmission cause corresponding variations in the current flowing in the output circuit of the gain control element 45. By connecting this output circuit across a portion of the resistance 35, the variable amplitude current from the element 45 causes a corresponding change in the potential of the control electrode 37. The nature of this change of the potential may be so taken as to compensate exactly for the change of the transmission level. For instance, if the transmission level falls, the grid electrode 37 will have its potential varied in the proper direction by an amount just sufllcient to compensate for the fallin the value of the direct current component in the transmission line. Similarly, a corresponding change may be made for rises in the transmission level.

The following mathematical calculation will serve to prove the statement that the gain control element 45 can be made to respond only to changes in the amplitude of the total current in the transmission system:

Let E=P cos 0 where E is the potential impressed upon the control electrode 47 and a is a quantity which includes the variable angle due to the variations in the frequency of the modulated carrier wave.

From the well known general equation giving the relation between grid potential and plate current namely etc., (Ki, K2 and K3 being constants) the following may be written as the approximate equation when the circuit is adjusted so that the electron discharge device functions as a demodulator:

I =K(P cos 9)' =-KP cos 0) Where K is any suitable constant. From trigonometry it follows that The term &1 represents the zero frequency component which is seen to be entirely independent of 0. Therefore, if P is a constant, the zero frequencycomponent is of constant amplitude and is independent of any variations of 0. It will be noted that the term cos 0 does not occur in the above equation which indicates that the side bands of frequencies in the immediate proximity to the unmodulated carrier frequency are not present.

By properly adjusting the circuit including the condenser 49, the current component represented by the second term of the above expression, namely P cos 0, may be filtered out. Thus, the current flowing in the plate circuit of the tube 45 which serves to modify the potential of-the grid 37 is made to vary directly as variations in the term P, or in accordance with variations in the amplitude of the modulated wave in the transmission line.

The optical system and the synchronizing arrangements employed in this system may be of any wclinvwn and approved it Fora better understanding of a complete system of this character, reference is made to the Patent No. 1,606,227 to Horton et al., Nov. 9, 1926.

What is claimed is:

1. The combination in an image transmission system of means for generating an alternating carrier current, means for modulating the frequency of said current in accordance with the tone values of a picture or object, means for transmitting and receiving the modulated current, means for varying the amplitude of the received current in accordance with its frequency, a light valve controlled by said variable amplitude current for producing an image of said picture or object, and means responsive only .to variations in the amplitude of the transmitted modulated wave for compensating for such varia tions.

2. In combination, means for producing an alternating carrier current, means for varying the frequency of said current in accordance with the characteristics of a picture or object, a transmission line for transmitting said variable frequency current, means for varying the amplitude of the transmitted current in accordance with the variations in frequency, optical means controlled by the variable amplitude current for producing an image of the picture or object, a demodulator responsive to ,the transmitted carrier current for producing an output current which is independent of the frequency changes in said carrier current and depends only upon a change in the amplitude of said carrier current, and means controlled by said output current to teoiinpensate for such variations in the ampli- 3. An electrical transmission system comprising means for transmitting a frequency modulated carrier wave, means for converting said frequency modulated carrier wave into an amplitude modulated wave, means for demodulating said amplitude modulated wave, and means responsive only to variations in the amplitude of the transmitted frequency modulated wave for compensating for such variations.

4. The combination in a signal transmission system of means for generating alternating car rier current, means for modulating the frequency of said current in accordance with the signal, means for transmitting and receiving the modulated current, means for producing a current the amplitude of which varies in acordance with the frequency variations of said received current, indicating means controlled by the resulting variable amplitude current, and means responsive only to variations in the amplitude of the transmitted modulated wave produced during transmission to compensate for such amplitude variations. I

5. The combination in an image transmission system of means for generating alternating carrier current, means for modulating the frequency of said current in accordance with the characteristics of a picture or object, means for transmitting and receiving the modulated current, means for producing a current the amplitude of which varies in accordance with the frequency variations of said received current, optical means controlled by said variable amplitude current for producing an image of the picture or object and means responsive only to variations in amplitude of the transmitted modulated wave produced during transmission to compensate for such amplitude variations.

6. Receiving apparatus for a frequency modulated signaling wave comprising repeating means under control of said wave, means controlled by current from said repeating means and operating in accordance with the frequency variations of said frequency signaling wave, and means controlling said repeating means and controlled by said wave for compensating for changes in amplitude of said wave to cause the amplitude of the output current of said repeating means to remain substantially independent of said amplitude variations.

7. The method of receiving a signaling carrier current modulated with respect to frequency which comprises utilizing a portion of the received carrier current to control a repeating device, utilizing the repeated current to reproduce the signal in accordance with said frequency characteristic, and utilizing another portion of the energy of the received carrier current to control the operation of the repeating device in such manner as to compensate for amplitude variations of the received carrier current whereby the current utilized in the reproduction of the signal is independent of said amplitude variations.

8. The method of producing images of pictures or other objects which comprises receiving a current modulated with respect to frequency in accordance with the tone values of the elemental areas of the picture or object, varying the amplitude of said received current in accordance with its frequency, introducing variations into said current of varying amplitude to compensate for the amplitude variations introduced due to the changing transmission characteristics of a transmission medium, and employing said compensated current of varying amplitude to control the production of an image of the picture or object.

9. An electro-optical image producing system comprising means for receiving a carrier current modulated with respect to frequency in accordance with the tone values of elemental areas of a picture or other object an image of which is to be produced, means for varying the amplitude of said received current in accordance with its frequency, means for introducing other variations into said current of varying amplitude to compensate for the amplitude variations introduced due to thechanging transmission characteristics of a transmission medium, and optical means 

